<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>

    <script>
        window.onload = () => {
            let sheng = document.getElementById("sheng");
            let shi = document.getElementById("shi");
            let xian = document.getElementById("xian");
            let change = document.getElementById("change")
            sheng.onchange = () => {
                let index_sheng = sheng.selectedIndex;
                let value_sheng = sheng.options[index_sheng].innerHTML;


                change.value += `${value_sheng}`


            }
            shi.onchange = () => {
                let index_shi = shi.selectedIndex;
                let value_shi = shi.options[index_shi].innerHTML;

                change.value += `${value_shi}`;
            }
            xian.onchange = () => {
                let index_xian = xian.selectedIndex;
                let value_xian = xian.options[index_xian].innerHTML;
                change.value += `${value_xian}`;

            }


        }
    </script>
</head>
<body>


<form action="">
    <select id="sheng">
        <option value="">请选择省</option>
        <option value="">河南</option>
        <option value="">广东</option>
    </select>
    <select id="shi">
        <option value="">请选择市</option>
        <option value="">郑州市</option>
        <option value="">广州</option>
    </select>
    <select id="xian">
        <option value="">请选择县</option>
        <option value="">叼毛</option>
        <option value="">天河区</option>
    </select>
    <input type="text" id="change" >


</form>
</body>
</html>